This invention relates to the manufacture of thermoplastics pipe. In particular, it is concerned with a method for making a length of thermoplastics pipe by the radial expansion of a pipe blank within a circular cross-section mould. The invention is especially suitable for use with PVC (polyvinyl chloride) plastics to make pipes for water-supply and distribution purposes.
It has previously been proposed to heat a pipe blank within a mould and expand the pipe radially by means of liquid under pressure, see eg British Patent 1432539. The expansion at the temperature specified in that specification causes orientation of the thermoplastics material, which gives an increased hoop strength in the finished product. In this manner, relatively high strength pipes can be formed using a minimum of thermoplastics, and the principle has been found particularly suitable for the manufacture of large diameter PVC pipes.
The finished pipe, which has a socket formed at one end, may be of the order of 10 meters long with a diameter of 450 mm. (The starting blank has of course a smaller diameter). Both the blank and the finished pipe need to be handled and located with accuracy if the desired quality of product is to be achieved.